In general, a tunneling technology is used to allow an IPv6 packet to cross an IPv4 network by configuring a tunnel, when the IPv6 packet must be transmitted from an IPv6 network to another IPv6 network via the IPv4 network. IPv4/IPv6 dual stack hosts and routers can perform tunneling through an IPv4 routing topology region by encapsulating an IPv6 datagram to an IPv4 packet.
Since IPv6 island networks appear at the beginning stage of IPv4 to IPv6 transition, the tunneling technology will be used to a great extent. Thus, many efforts have been actively made for standardization of the tunneling technologies among various transition technologies. As a result, a variety of tunneling technologies have been proposed as the standard tunneling technology. Representative ones of those tunneling technologies include ‘Configured tunnel’, ‘6to4’, ‘6over4’ and ‘Intra-Site Automatic Tunnel Addressing Protocol (ISATAP)’.
To provide the IPv6 communication between terminals on an IPv4 network, a control tunnel and a direct tunnel are used. The control tunnel is similar to existing Tunnel Broker or Teredo, but includes a user group management module that allows peer-2-peer data communication such as MSN or SkyPe, which is being widely used. The control tunnel provides a direct tunnel between the terminals so that data can be directly transmitted and received therebetween, which is very advantageous in data transmission.
The most important factor in setting the direct tunnel is a network address translator (hereinafter, referred to as an NAT). When terminals are located currently on a network, the terminals may be assigned public IP addresses by an Internet Service Provider (ISP) and may be assigned private IP addresses by the NAT. To overcome the lack of IP addresses, the NAT uses a source NAT (SNAT), dynamic NAT (DNAT)-port forwarding, and a DNAT-load distribution. As for the DNAT-port forwarding, a server to operate is placed on a specific private network, and access thereto is allowed only for a specific user who knows the specific private network on which the service is placed. In the case of the DNAT-load distribution, the load of TCP/UDP traffic is distributed to a plurality of servers on an internal network. Such an NAT is connected to an external network by using a public IP address assigned by the IPS, and assigns a private IP address to each of terminals attached to the NAT. The NAT enables communication by maintaining mapping information of a private IP address/port number, and an IP address/port number of an external network. Here, the DNAT is not used for general network operation because the NAT must be controlled in advance for a specific purpose.
The NAT allows terminal host A and terminal host B on its private network having IP address 192.168.2.0 to be connected simultaneously with an external network by using one public IP address 100.100.100.50 allocated to a user from the IPS. In detail, when an NAP router set to a default gateway of the terminal A and terminal B detects through routing that destinations of IP packets sent from the terminal A and terminal B is on the external network, the NAT router buffers IP addresses and port numbers of the transmitting terminals. Then, the NAT router respectively translates the IP addresses and port numbers to the public IP address 100.100.100.50 and new port numbers of 60000 or greater, and transmits to the IPS. A router of the IPS having received the packets considers those packets to be sent from two different processes of the same terminal 100.100.100.50, and thus can transmit the packets to another Internet network without limitations.
Also, when the packets are transmitted from the ISP, the NAT router searches the mapping table including the port numbers and internal network addresses to detect an IP address and a port number of a destination terminal, and transmits to the corresponding terminal. Thus, a plurality of terminals on the private network can use a public network by sharing one IP address provided by the ISP.
The SNAT is available only when clients on the private network access external servers. In the case where file transfer protocol (FTP) servers are installed in the private network, external terminals send packets by using a public IP address allocated to the NAT and a well-known FTP port number such as 20 and 21 since an IP address of the server on the internal network is unknown to the external terminals. The NAT router having received such a packet may consider the destination IP address and port number of the received packet to be for an FTP server installed in the NAT itself, and thus the received packet cannot be transmitted to another FTP server on the private network.
If an external user has already been informed of a specific FTP server in the private network, the user does not use a well-known FTP port number 21, but serves to send a packet whose destination is port number 3000 that is intentionally allocated to the FTP server of an internal network. In detail, a destination IP address of the packet is a public IP address of the NAT, whereas a destination port number is port number 3000, not the well-known FTP port number.
Accordingly, when receiving the FTP packet with the known port number 3000 that the NAT router knows, the NAT router changes the IP address into an IP address of the FTP server installed on its private network, and also changes the destination port number of the packet into port number 21, and then transmits. In such a manner, the NAT router makes the FTP server on the private network accessible by clients on an external network.
However, other users who are not informed of a specific port number of the specific server cannot use the aforementioned function, and to enable this function, information must be stored in the NAT in advance.
The current communication service is based on IPv4, and many efforts are being made to evolve into an IPv6-based service. However, in actuality, there are not many IPv6-based services, and thus technologies for transition from IPv4 to IPv6 are required. Even though many technologies have been conventionally proposed, those conventional technologies require many changes in equipment on a network.